Game Logic and the delay sensor: how to make "delay" a random number
You can use a python script to change the delay to a different random value each time it fires.

import bge
import random

cont = bge.logic.getCurrentController()

sens = cont.sensors['Delay']

sens.delay = random.randint(60,120)


Add the script to a text block in blender and connect a python controller to the delay sensor.